More specifically, the invention relates to turbofan engines, described for example in document WO 2006/123035, of the type comprising, around a longitudinal axis:                a nacelle provided with an outer nacelle cowl and enclosing a fan generating the cold stream and a central generator generating the hot stream;        an annular cold-stream duct formed around said central hot-stream generator;        an outer fan cowl bounding said annular cold-stream duct in the region of said outer nacelle cowl;        a cold-stream outlet orifice whose edge, which forms the trailing edge of said nacelle, is defined by said outer nacelle cowl and by said outer fan cowl converging toward one another until they meet up;        an inner fan cowl, bounding said annular cold-stream duct in the region of said central hot-stream generator, and forming a projection toward the rear of said turbofan engine outside said cold-stream outlet orifice; and        a cold-stream nozzle throat which is formed between said inner fan cowl and said outer fan cowl.        
To make it possible in such a turbofan engine to minimize the performance losses due to the friction of said cold stream in said annular cold-stream duct, it is common practice to optimize the shape and the surface of said outer fan cowl, and also the shape and surface of said inner fan cowl. In particular, care is taken to ensure that the surfaces of said fan cowls are as smooth as possible.
Moreover, it is known (see, for example, European document EP 1 031 510) that, owing to the difference in the pressures at said cold-stream outlet orifice between said cold stream and the external aerodynamic flow around said nacelle, an alternating succession of supersonic speed zones and subsonic speed zones arises in said cold stream to the rear of said nozzle throat, the transitions between the supersonic speed zones and the subsonic speed zones being abrupt, non-progressive and without intermediate speed values and resulting from normal shocks, that is to say shocks which are virtually at right angles to the flow of said cold stream. As a result, said cold stream is the source of a shockwave propagating at the rear of said turbofan engine and having the shape of a broken line whose segments, (termed “characteristics” in aeronautical terminology) have a small inclination with respect to the flow of said cold stream and are reflected alternately on the slip surface between said cold stream and the external aerodynamic flow around said nacelle and on the slip surface between said cold stream and said hot stream.
Such a shockwave having normal shocks not only generates considerable noise (known as “shock cell noise”) but also negatively impacts the performance of the turbofan engine and therefore that of the aircraft carrying it.